hw/block/fdc-sysbus: Always mark sysbus floppy controllers as not having DMA

The sysbus floppy controllers (devices sysbus-fdc and sun-fdtwo)
don't support DMA.  The core floppy controller code expects this to
be indicated by setting FDCtrl::dma_chann to -1.  This used to be
done in the device instance_init functions sysbus_fdc_initfn() and
sun4m_fdc_initfn(), but in commit 1430759ec3e we refactored this code
and accidentally lost the setting of dma_chann.

For sysbus-fdc this has no ill effects because we were redundantly
also setting dma_chann in fdctrl_init_sysbus(), but for sun-fdtwo
this means that guests which try to enable DMA on the floppy
controller will cause QEMU to crash because FDCtrl::dma is NULL.

Set dma_chann to -1 in the common instance init, and remove the
redundant code in fdctrl_init_sysbus() that is also setting it.

There is a six-year-old FIXME comment in the jazz board code to the
effect that in theory it should support doing DMA via a custom DMA
controller.  If anybody ever chooses to fix that they can do it by
adding support for setting both FDCtrl::dma_chann and FDCtrl::dma.
(A QOM link property 'dma-controller' on the sysbus device which can
be set to an instance of IsaDmaClass is probably the way to go.)

Fixes: 1430759ec3 ("hw/block/fdc: Extract SysBus floppy controllers to fdc-sysbus.c")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/958
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20220505101842.2757905-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

1 files changed, 11 insertions, 5 deletions

diff --git a/hw/block/fdc-sysbus.c b/hw/block/fdc-sysbus.c
index 57fc877..86ea51d 100644
--- a/hw/block/fdc-sysbus.c
+++ b/hw/block/fdc-sysbus.c
@@ -94,18 +94,14 @@ static void fdctrl_handle_tc(void *opaque, int irq, int level)
     trace_fdctrl_tc_pulse(level);
 }
 
-void fdctrl_init_sysbus(qemu_irq irq, int dma_chann,
-                        hwaddr mmio_base, DriveInfo **fds)
+void fdctrl_init_sysbus(qemu_irq irq, hwaddr mmio_base, DriveInfo **fds)
 {
-    FDCtrl *fdctrl;
     DeviceState *dev;
     SysBusDevice *sbd;
     FDCtrlSysBus *sys;
 
     dev = qdev_new("sysbus-fdc");
     sys = SYSBUS_FDC(dev);
-    fdctrl = &sys->state;
-    fdctrl->dma_chann = dma_chann; /* FIXME */
     sbd = SYS_BUS_DEVICE(dev);
     sysbus_realize_and_unref(sbd, &error_fatal);
     sysbus_connect_irq(sbd, 0, irq);
@@ -138,6 +134,16 @@ static void sysbus_fdc_common_instance_init(Object *obj)
     FDCtrlSysBus *sys = SYSBUS_FDC(obj);
     FDCtrl *fdctrl = &sys->state;
 
+    /*
+     * DMA is not currently supported for sysbus floppy controllers.
+     * If we wanted to add support then probably the best approach is
+     * to have a QOM link property 'dma-controller' which the board
+     * code can set to an instance of IsaDmaClass, and an integer
+     * property 'dma-channel', so that we can set fdctrl->dma and
+     * fdctrl->dma_chann accordingly.
+     */
+    fdctrl->dma_chann = -1;
+
     qdev_set_legacy_instance_id(dev, 0 /* io */, 2); /* FIXME */
 
     memory_region_init_io(&fdctrl->iomem, obj,